Tarr Chronicles and Dark Horizon version has just been released as well as an SDK for the BRU Engine (Which is used by both games). The main difference between and is the mod support and plugin support.

You can now create mods using the BRU Engine SDK and use them with Tarr Chronicles and Dark Horizon (Note: Mods made for one game may not work with the other). There are currently no known limitations with the mod support except profiles. Profiles are currently global rather than being local to a mod/game(Profiles will become local with the next version). Total Conversions are possible.

You can also create plugins(In C++ or Delphi) for both games (They can either be global or local to a mod) but at this time they only give access to LUA and not direct access to Engine variables.

Both Plugin and Mod Support will be extended over 2012 and beyond. This is the Beginning, not the End.

- BRU Engine Official Site